The time for obsessing over key state opinion polls was over as the greatest political show entered its thrilling finale after an 18-month roller-coaster ride that exposed the nation's bitter polarization.

The most expensive campaign in US history saw an estimated $6 billion unleashed by the rival camps in a fight to win over that thin sliver of the electorate that could swing either way.

After a frantic burst of last-minute campaigning, voters were deciding whether to re-elect Obama despite the plodding economy or hand the reins to Romney, who has vowed a return to prosperity through smaller government.

They will also cast judgment on a third of the Democratic-led Senate and the entire Republican-run House of Representatives. But with neither chamber expected to change hands, the current political gridlock will likely linger.

Obama, 51, led his Republican opponent by a whisker heading into polling day as he sought to defy historical precedent that suggests American presidents fail to win second four-year terms at times of high unemployment.

Romney, 65, a former Massachusetts governor blasted by critics as a rich plutocrat indifferent to middle class pain, would make history as the first Mormon president and promises to ignite economic growth and job creation.

Casting his vote in Belmont, Massachusetts, with wife, Ann, Romney said he felt "very, very good" about his election prospects, but his decision to make unusual polling day trips to Ohio and Pennsylvania betrayed nerves to some.

Both men made their final, heartfelt plea to voters in late night rallies on Monday attended by fervent supporters.

"Tomorrow, from the granite of New Hampshire to the Rockies of Colorado, from the coastlines of Florida, to Virginia's rolling hills, from the valleys of Ohio to these Iowa fields, we will keep America moving forward," Obama said.

Speaking in Iowa, the state that first nurtured his White House dreams in 2008, a single tear rolled down the president's face as he wrapped up what was -- win or lose -- his last-ever campaign event.

Romney put an exclamation mark on his campaign with his own, rowdy late night rally at a sports arena in New Hampshire, where he launched his own White House crusade almost 19 months ago.

"Tomorrow is a moment to look into the future and imagine what we can do, to put that past four years behind us and build a new future," he said.

Voters will also weigh in on more than 170 state-wide ballot issues for everything from gay marriage to marijuana legalization and abortion.

A dispiriting White House race, so different from Obama's euphoric drive for hope and change in 2008, produced the election both sides expected -- a frantic scrap for thin victory margins in 10 or so swing states.

The US presidential election is not directly decided by the popular vote, but require candidates to pile up a majority -- 270 -- of 538 electoral votes awarded state-by-state on the basis of population.

A candidate can therefore win the nationwide popular vote and still be deprived of the presidency by falling short in the Electoral College.

Obama, the first African American president, was clinging to a last line of defense in the Midwestern states of Ohio, Wisconsin and Iowa, which would, in conjunction with safe Democratic states, guarantee him re-election.

Romney led by 1.5 percent in the biggest swing state, Florida, and in North Carolina, which Obama won by just three percent, or 14,000 votes, in 2008.

Romney aides predicted a surge of enthusiasm for the Republican would confound state polls, which they said overestimated the likely Democratic turnout and did not register the undercurrent of antipathy for Obama.

"It's going to be a higher than normal turnout for sure," said Romney campaigner Chris Redder as he distributed sample ballots to voters in Falls Church, Virginia, indicating which Republican boxes they should tick.

"I consider this an important election," he told AFP. "It's two visions of America -- more personal responsibility versus more intrusive government, and pro-life versus what I'd say pro-abortion."

Down in Florida, at an auditorium near Little Havana, people started showing up four hours early to cast their ballots.

The central message of Obama's campaign has been that he saved America from a second Great Depression after the economy was on the brink of collapse when he took over from Republican president George W. Bush in 2009.

He claims credit for ending the war in Iraq, saving the US auto industry, killing Osama bin Laden, offering almost every American health insurance, and passing the most sweeping Wall Street reform in decades.

Romney sought to mine frustration with the slow pace of the economic recovery and argued that the president was out of ideas and had no clue how to create jobs, with unemployment at 7.9 percent and millions out of work.

No president since World War II has been elected with the unemployment rate above 7.4 percent, and Obama is hoping to avoid the fate of a host of European leaders who paid for the economic crisis with their jobs.